A souterrain, the term used for the stone-lined or rock-cut tunnels that early medieval communities built beneath their settlements, usually announces itself through its masonry. At Glennafosha, there is none visible. What remains is simply a depression in the earth, L-shaped and sizeable, the landscape sagging quietly where something once ran underground.

The hollow sits within a ringfort, the type of enclosed farmstead that was the dominant form of rural settlement in Ireland from roughly the fifth to the twelfth centuries. Ringforts typically comprised a circular area bounded by an earthen bank and ditch, within which a family would have lived and kept animals. Souterrains were a common feature of such sites, their precise function still debated, though most archaeologists understand them as places of refuge, cool storage, or both. The Glennafosha example is L-shaped, with a longer axis of about 11.4 metres running northeast to southwest, and a shorter arm of 7.7 metres extending northward from the northeastern end. These are not modest dimensions; the combined passage would have formed a substantial underground space. The absence of visible stonework suggests either that the structure was cut directly into the subsoil rather than built up with dressed stone, or that whatever lining it once had has long since collapsed inward and been absorbed into the surrounding earth.